1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical recording medium drive employing the same.
2. Description of the Prior Art
An optical pickup apparatus employed for an optical recording medium drive such as an optical disk drive is adapted to record or read information in or from an optical recording medium such as an optical disk or detect a servo signal with a laser beam.
FIG. 20 schematically illustrates a conventional optical pickup apparatus disclosed in Japanese Patent Laying-Open Gazette No. 3-76035 (1991). This optical pickup apparatus performs tracking servo control by the three-beam method.
Referring to FIG. 20, symbols X, Y and Z denote the radial direction of an optical disk 1, the track direction of the optical disk 1, and a direction perpendicular to the disk plane of the optical disk 1 respectively.
A semiconductor laser device 102 emits a laser beam B in the direction Z. The beam B emitted from the semiconductor laser device 102 enters a diffraction grating 103. FIG. 21 is a plan view of the diffraction grating 103. The diffraction grating 103 has a grating surface 103a formed by unevenness of regular pitches. The grating surface 103a divides the incident laser beam B into three beams, i.e., a 0th order diffracted beam (main beam), a +1st order diffracted beam (subbeam) and a -1st order diffracted beam (subbeam), and transmits the same through a transmission-type holographic optical element 104.
Referring to FIG. 20, an objective lens 105 condenses the three beams transmitted through the transmission-type holographic optical element 104 on the optical disk 1. FIG. 22 is a model diagram showing the condensed states on the recording plane of the optical disk 1. As shown in FIG. 22, the 0th order diffracted beam is condensed on a track surface TR of the recording plane as a main spot MO, and the .+-.1st order diffracted beams are condensed on both sides of the main spot MO as subspots S1 and S2 respectively.
The transmission-type holographic optical element 104 diffracts three returned beams (reflected beams) from the main spot MO and the subspots S1 and S2 in a plane substantially including the directions X and Z, so that a photodetector 106 detects these returned beams.
FIG. 23 is a typical plan view showing an exemplary photodetector 106. This photodetector 106 includes a photodetection part 106a provided on the central portion for performing focus servo control with the astigmatism method and photodetection parts 106b and 106c provided on both sides of the photodetection part 106a for performing tracking servo control with the three-beam method. The returned beam corresponding to the main spot MO enters the central portion of the photodetection part 106a while the returned beams corresponding to the subspots S1 and S2 enter the photodetection parts 106b and 106c respectively.
The aforementioned optical pickup apparatus performs tracking control in the following manner: As shown in FIG. 22, the track surface TR recording information is different in light reflectance from a non-track surface. When the photodetection parts 106b and 106c detect the returned beams from the subspots S1 and S2, the returned beams from the two subspots S1 and S2 entering the two photodetection parts 106b and 106c are equal in light intensity to each other if the main spot MO excellently tracks the track surface TR to be reproduced. If the main spot MO deviates to either side of the track surface TR, on the other hand, the photodetection part 106a or 106b relatively largely detects the light intensity of the returned beam from one of the subspots S1 and S2. With output signals E and F from the photodetection parts 106b and 106c, therefore, the following tracking error signal TE is obtained:
TE=E-F PA1 TE=E-F
The optical pickup apparatus performs excellent tracking control when the tracking error signal TE is zero, and detects deterioration of the tracking state as the value of the tracking error signal TE increases.
When detecting the tracking error signal TE, the optical pickup apparatus moves the objective lens 105 in the radial direction (the direction X), for correcting the condensed positions of the main spot MO and the subspots S1 and S2 on the track surface TR of the optical disk 1.
FIG. 24A is a typical sectional view showing the condensed states of diffracted beams B1 and B2 diffracted by the diffraction grating 103, and FIG. 24B shows typical plan views of the objective lens 105. As shown in FIG. 24A, the diffracted beam B1 diffracted by the diffraction grating 103 in the +1st order direction passes through the objective lens 105, to be condensed as the subspot S1. The diffracted beam B2 diffracted in the -1st order direction passes through the objective lens 105, to be condensed as the subspot S2.
Referring to FIG. 24B, the grating surface 103a of the diffraction grating 103 is formed to be larger than the laser beam B, as shown in FIG. 20. Therefore, the laser beam B incident on the grating surface 103a is diffracted over a region wider than an aperture 105a of the objective lens 105, to result in regions B1a and B2a not entering the aperture 105a of the objective lens 105.
When the optical pickup apparatus performs a tracking operation in this state and moves the objective lens 105 in the direction X (the radial direction of the optical disk 1), the incident states of the diffracted beams B1 and B2 on the objective lens 105 change from those on the left to those on the right in FIG. 24B. The ratios of the diffracted beams B1 and B2 entering the aperture 105a of the objective lens 105 reduce following movement of the objective lens 105. Therefore, the light quantities of the subspots S1 and S2 reduce on the recording plane 1a of the optical disk 1, to result in reduction of the light quantities of the returned beams from the subspots S1 and S2 entering the photodetection parts 106b and 106c. When the objective lens 105 is moved during the tracking operation, therefore, the output of the tracking error signal TE disadvantageously reduces.
FIG. 25 is a model diagram for illustrating the diffracted state of the beam B diffracted by the diffraction grating 105. Referring to FIG. 25, a light source 200 forms an emissive end of the semiconductor laser device 102, so that the laser beam B emitted from this light source 200 is condensed on the recording plane 1a of the optical disk 1 as the two subspots S1 and S2. The transmission-type holographic optical element 104 is omitted in FIG. 25.
The grating surface 103a diffracts the laser beam B emitted from the light source 200 at least in the +1st order direction and the -1st order direction. In the laser beam B, the +1st order diffracted partial beam of a partial beam BE1 passes through the objective lens 105, to be condensed as the subspot S1. The +1st order diffracted partial beam of a partial beam BE2 passes through a part beyond the objective lens 105, not to be condensed on the subspot S1.
On the other hand, the -1st order diffracted partial beam of a partial beam BE3 passes through the objective lens 105, to be condensed on the subspot S2. Further, the -1st order diffracted partial beam of a partial beam BE4 passes through a part beyond the objective lens 105, not to be condensed on the subspot S2.
When an optical axis LP passing through the peak of the light intensity distribution of the laser beam B aligns with a central axis ZO passing through the center of the objective lens 105, the light quantities of the partial beams BE1 and BE3 condensed on the subspots S1 and S2 respectively are equal to each other. Therefore, the correct tracking state can be detected by detecting the difference between the light quantities of the returned beams from the two subspots S1 and S2.
However, the optical axis LP of the laser beam B may deviate from the central axis ZO of the objective lens 105 due to a locational error of the semiconductor laser device 102 or the emission property of the laser beam B. When the optical axis LP deviates from the central axis ZO, the partial beams BE1 and BE3 are condensed on the two subspots S1 and S2 in non-uniform light quantities.
FIGS. 26A and 26B illustrate light intensity distribution states of the laser beam B in a section taken along the line Q--Q in FIG. 25. In FIGS. 26A and 26B, a symbol 2R denotes the diameter of the partial beam incidenting into the objective lens 105 within the +1st and the -1st order diffracted beams. The optical axis LP aligns with the central axis ZO in FIG. 26A, while the former deviates from the latter in FIG. 26B. FIG. 26A shows the light quantities corresponding to the partial beams BE1 and BE2 in regions (E1+E2) and (E3) respectively. Further, the light quantities corresponding to the partial beams BE3 and BE4 are shown in regions (E1+E3) and (E2) respectively.
As shown in FIG. 26A, the light quantity (the region (E1+E2)) of the partial beam BE1 condensed on the subspot S1 is equal to the light quantity (the region (E1+E3)) of the partial beam BE3 condensed on the subspot S2 when the optical axis LP aligns with the central axis ZO.
When the optical axis LP deviates from the central axis ZO, on the other hand, the light quantities of the partial beams BE1 and BE3 condensed on the subspots S1 and S2, which are shown in regions (E1+E20) and (E1+E30) respectively, differ from each other. Thus, the tracking error signal TES based on the returned beams from the two subspots S1 and S2 is so offset that it is difficult to detect the correct tracking state.
FIG. 27 schematically illustrates another conventional optical pickup apparatus. This optical pickup apparatus is adapted to perform tracking servo control and focus servo control with the three-beam method and the astigmatism method respectively.
Referring to FIG. 27, a laser beam 112 emitted from a semiconductor laser device 121 passes through a transmission-type diffraction grating 123 to be divided into three beams, i.e., a 0th order diffracted beam (main beam) and +1st order diffracted beams (subbeams) and transmitted through a transmission-type holographic optical element 124.
An objective lens 116 condenses the three beams transmitted through the transmission-type holographic optical element 124 on an optical disk 1 as a main spot MO and subspots S1 and S2 located on both sides thereof. An actuator 140 supports the objective lens 116 to be movable in the radial direction (the X-axis direction) of the optical disk 1 for a tracking operation and to be movable in the Y-axis direction for a focus operation.
FIG. 28 illustrates the main spot MO and the subspots S1 and S2 formed on the optical disk 1. As shown in FIG. 28, the optical system of the optical pickup apparatus is so adjusted that the main spot MO scans a track TR to be reproduced and the subspots S1 and S2 scan both sides of the main spot MO slightly over the track TR.
The transmission-type holographic optical element 124 diffracts three returned beams (reflected beams) from the optical disk 1, so that a signal detection photodiode 133 detects the same.
FIG. 29 is a typical plan view showing an exemplary signal detection photodiode 133. This signal detection photodiode 133 includes photodetection parts 150a to 150d provided on the central portion for performing focus servo control with the astigmatism method and photodetection parts 150e and 150f provided on both sides of the photodetection parts 150a to 150d for performing tracking servo control with the three-beam method. The returned beam (main beam) corresponding to the main spot MO enters the central portion of the photodetection parts 150a to 150d, while returned beams (subspots) 112a and 112b corresponding to the subspots S1 and S2 enter the photodetection parts 150e and 150f respectively.
On the basis of detection signals E and F from the photodetection parts 150e and 150f of the signal detection photodiode 133 receiving the returned beams (subbeams) 112a and 112b, the optical pickup apparatus performs the tracking operation in the following manner:
FIG. 30 is a circuit diagram showing respective parts of an optical disk drive comprising the optical pickup apparatus 100 performing the tracking operation. Referring to FIG. 30, the photodetection parts 150e and 150f of the signal detection photodiode 133 of the optical pickup apparatus 100 output the detection signals E and F to an E-F processing part 155 provided on a driving circuit part 154 of the optical disk drive. With the detection signals E and F received from the photodetection parts 150e and 150f, the E-F processing part 155 obtains the following tracking error signal TE:
The tracking error signal TE is inputted in an operational amplifier 158 of a servo circuit 157 through a low-pass filter 156, amplified and thereafter supplied to a tracking coil 142 of the actuator 140 of the optical pickup apparatus 100.
As shown in FIG. 27, the actuator 140 supports the objective lens 116 to be movable in the radial direction (the X-axis direction) of the optical disk 1. The actuator 140 comprises a holder 141 for holding the objective lens 116, the tracking coil 142 connected to the holder 141 to be movable in the radial direction, and a permanent magnet 144 separating from the tracking coil 142. When a driving voltage is applied to the tracking coil 142, the actuator 140 moves the objective lens 116 in the X-axis direction by electromagnetic force caused between the tracking coil 142 and the permanent magnet 144.
When the main spot MO formed on the optical disk 1 effectively tracks the track TR to be reproduced in FIG. 28, the returned beams 112a and 112b from the two subspots S1 and S2 enter the photodetection parts 150e and 150f in equal light intensity. Therefore, the tracking error signal TE outputted from the E-F processing part 155 is zero and no driving voltage is applied to the tracking coil 142 of the actuator 140. Thus, the objective lens 116 maintains its state.
When the main spot MO deviates to either side of the track TR to be reproduced, on the other hand, the light intensity of the returned beam 112a or 112b from the subspot S1 or S2 increases. Thus, the detection signals E and F from the photodetection parts 150e and 150f differ from each other. Therefore, the E-F processing part 155 outputs the tracking error signal TE, which in turn is amplified by the operational amplifier 158 of the servo circuit 157 so that a driving voltage is applied to the tracking coil 142 and the actuator 140 radially moves the objective lens 116 for correcting the position of the main spot M1.
In recent years, miniaturization of such an optical pickup apparatus 100 is strongly desired, and the respective elements thereof are miniaturized with reduction of the diameter of the objective lens 116. In an assembling step for the optical pickup apparatus 100, therefore, it is difficult to correctly align the objective lens 116 with the optical path of the laser beam 112.
FIG. 31 is a typical plan view showing an incident state of the laser beam 112 on the objective lens 116. In the optical pickup apparatus 100, the semiconductor laser device 121, the diffraction grating 123 and the transmission-type holographic optical element 124 are integrated into a unit independently of the objective lens 116, and these units are assembled with each other in alignment. In assembling, therefore, the optical axis of the objective lens 116 may deviate from those of the two subbeams 112a and 112b of the laser beam 112 by d along the radial direction (the X-axis direction) of the optical disk 1, as shown in FIG. 31.
Such deviation d in the mounting position of the objective lens 116 results in the following disadvantage: The optical disk drive moves the objective lens 116 by a constant distance in the radial direction of the optical disk 1 in order to search the program for a tune recorded in the optical disk 1, for example. If the optical axis of the objective lens 116 deviates from those of the subbeams 112a and 112b of the laser beam 112 by d in assembling as shown in FIG. 31, however, the subbeams 112a and 112b pass through the objective lens 116 in different light quantities following movement of the objective lens 116 for the program search, in response to the direction of movement. The light quantities of the subbeams 112a and 112b passing through the objective lens 116 extremely reduce following movement of the objective lens 116 in one direction, and hence the output of the tracking error signal TE based on the subbeams 112a and 112b passing through the objective lens 116 reduces to hinder a effective tracking operation.
FIG. 32 illustrates changes of the tracking error signal TE following movement of the objective lens 116. Referring to FIG. 32, the horizontal axis shows the direction and the distance of movement of the objective lens 116, and the vertical axis shows the tracking error signal TE. When the center of the objective lens 116 aligns with the optical axis of the laser beam 112 in the radial direction of the optical disk 1, symmetrical distribution TEO of the tracking error signal TE is obtained following movement of the objective lens 116, as shown by a dotted line in FIG. 32. When the center of the objective lens 116 deviates from the optical axis of the laser beam 112, on the other hand, asymmetrical distribution TE1 of the tracking error signal TE is obtained depending on the direction of movement of the objective lens 116, as shown by a solid line. The tracking error signal TE reduces below an output value A necessary for tracking on a position of movement of the objective lens 116, to hinder correct program search.
In general, therefore, an offset circuit 159 is provided on one input side of the operational amplifier 158 of the servo circuit 157, in order to correct the deviation of the objective lens 116 from the optical axis of the laser beam 112. In the optical pickup apparatus 100 built into the optical disk drive, the offset circuit 159 corrects the deviation of the objective lens 116 as follows:
The offset circuit 159 moves the objective lens 116 along the radial direction toward the center and the outer periphery respectively by prescribed distances of 400 lm, for example, and detects the voltages of the tracking error signal TE. If the center of the objective lens 116 deviates from the optical axis of the laser beam 112, the tracking error signal TE1 exhibits different voltages following movement of the objective lens 116 toward the center and the outer periphery, as shown in FIG. 32. Therefore, the movement origin position (the position of the objective lens 116 performing no tracking operation) is moved for equalizing the voltages of the tracking error signal TE in movement of the objective lens 116 toward the center and the outer periphery.
The resistance value of a variable resistor 160 of the offset circuit 159 is adjusted and a driving voltage is applied to the tracking coil 142 for moving the movement origin position of the objective lens 116 in the radial direction of the optical disk 1. Further, the objective lens 116 is moved from the movement origin position along the radial direction of the optical disk 1 toward the center and the outer periphery by prescribed distances respectively, for detecting the current values of the tracking error signal TE. Adjustment of the variable resistor 160 of the offset circuit 159 is ended when the detected values of the tracking error signal TE are equal to each other in movement toward the center and the outer periphery. Thus, the deviation of the objective lens 116 from the optical axis of the laser beam 112 in the radial direction of the optical disk 1 can be corrected.
However, the optical pickup apparatus 100 may be independently manufactured and put on the market by a manufacturer different from that for the optical disk drive employing the same. In this case, therefore, the manufacturer for the optical disk drive or the like must adjust the deviation of the objective lens 116 of the optical pickup apparatus 100 with complicated assembling and adjusting operations.